Why can't I parent objects in my .blend?
Set outliner display mode to Scene

I wonder if this is the outliner display mode being viewlayer rather than scene as default. Was a source of confusion for me the other day as  parent relationship works, but in viewlayer display doesn't show as hierarchy like we are used to in 2.79 and prior.  

In your file cube is a child of another cube that is a child of the empty. Can drag around in outliner after setting display mode to scene and parenting works as in previous versions with drag and drop.